Yes, H. The thing is there already was a theoretical border in terms of the different legal systems of Scotland and England and it DID run straight due east from the mouth of the Tweed! In other words, if a trawlerman murdered his skipper at sea north of the line, he would be tried in Scotland according to the Scottish system and vice versa, if the crime were south of the line.
The key element is, in my view, the fact that the ocean to the west of the Orkneys/Shetlands - just as is the case with the Scottish mainland - is (a) the North Atlantic. Only the eastern coasts are, of course, adjoining (b) the North Sea. Area 'a' has promise whilst Area 'b' is fading, though still producing the goods. I'm still convinced this was a key factor in the Westminster parties' unanimous decision to keep Scotland within the Union.